Hydraulically powered power unit

ABSTRACT

An hydraulically powered multiplex pump having at least three pumping units, each operable in a cycle including suction, precompression and discharge phases, with the cycles being out of phase with one another, whereby simultaneous performance of these functions results in a substantially constant pressure and flow of both the pumped fluid and the power fluid. Separate power and cycle control circuits, which may employ different fluids, are provided. Control valve assemblies, each including two sleeve valves communicating with a common chamber, are operated by the control circuit fluid to condition power circuit flow for the various phases of the pumping cycle. The power end of the pumping units includes power cylinders which may be fluid interconnected at their rod ends so that operations in each power cylinder affect functions in the other chambers, and provision is made for automatic correction of errors in stroke length.

ited States Patent Cole [4 May 16, 1972 [54] HYDRAULICALLY POWERED POWER UNIT 7 Primary Examiner-Robert M. Walker An -Bum D Sweck & Math' [72] Inventor: Clinton W. Cole, Duncan, Okla. omey 5 Dane er [73] Assignee: Halliburton Company, Duncan, Okla. ABSTRACT [22] Filed: Jan. 21, 1971 An hydraulically powered multiplex pump having at least three pumping units, each operable in a cycle including suc- [211 App! 108382 tion, precompression and discharge phases, with the cycles Related Application Data being out of phase with one another, whereby simultaneous performance of these functions results in a substantially conl Dlvlslon 0f 386,687, i969- stant pressure and flow of both the pumped fluid and the power fluid. Separate power and cycle control circuits, which [52] [1.8. CI. ..9l/4ll R,4l7/346 may employ different fluids are provided. Comm] valve [51] Int. Cl ..Fbl1/00,Fl5b 13/00, F04b 17/00, semblies, each including two Sleeve valves communicating F04!) /09 with a common chamber, are operated by the control circuit [58] new of Search 5? fluid to condition power circuit flow for the various phases of 91/ the pumping cycle. The power end of the pumping units includes power cylinders which may be fluid interconnected at [56] References Cned their rod ends so that operations in each power cylinder affect UNITED STATES PATENTS functions in the other chambers, and provision is made for au- 3 26 650 2/1969 J 91/4] 1 R tomatic correction of errors in stroke length.

,4 enney 3,505,929 4/1970 Coppola et al. 91/41 I R X 18 Claims, 18 Drawing Figures FP O l 62 5P 5| :9 55 44 r 4 1 5e 42 51 K 26 34 PR O E 4 a 50 4a 5/ 40 4: 54 e@ I 52 I M 65 30 I I l 1 ill J] 11] y sum 01 or 16 PKTENTEIJIAY 1s m PITENTEUM 1: an

sum 03 or 16 WENTEDMY 1a In 3.862.652

SHEET 0 0F 16 mmtcmnm 3,662,652

sum as 0F 16 SHEET 09 OF 16 FIG.9A2

AFFECTED VALVES NONE OPERATION OPENED Y CLOSED OPENED I VALVE COMMON PRESSURIZED TANKEO TANKEO VALVE cLosizok PKTENTEMHBM 3.862.652

sum 12 or 15 TABLE 82 I I I AFFECTED I I VALVE common VALVE OPERATION v VALVES B PRESSURIZED 31 OP NED 21 I 21 CLOSED A 'TANKED v 7 2n OPENED ID a IPR TANKED ID 0350 NONE PKTENTEDMAY 1s SHEET 130? 16 OONDITION I STROKE CONTROL VALVE PRESSURIZEO TANKED VALVES IWAFFECTED TABLE 5 TRIPPED VALVE CONTROL CIRCUIT CONDITIONED FLOW ASSEMBLY POWER CIRCUIT VALVE ASSEMBLY FIG .953

1uur16 SHEET P'ATE'N'TEDMAY 16 672 TABLE c cco coum- CYL- FUNC- VALVE non mom non PATENTEDMAY 16 m2 sum 15 or 16 FIG.9C2

AFFECTED VALVES NONE OPERATION OPENED CLOSED OPENED CLOSED VALVE CONDITION PRESSURIZEO TANKED TANKED VALVE HYDRAULICALLY POWERED POWER UNIT INDEX Column ABSTRACT OF THE DISCLOSURE FP BACKGROUND OF THE INVENTION 1 OBJECTS AND SUMMARY OF THE INVEN- TION 2 THE DRAWINGS 3 DETAILED DESCRIPTION 4- General Summary 4- Detailed Structure 6 The Control Valve Assembly 6 The Control Valve Manifolds 8 The Precompression Valve 10 The Power Fluid & Control Fluid Circuits 12 Miscellaneous Valve Structures 14 The Pumping Cycle 1 The Automatic Stroke Control SUMMARY OF ADVANTAGES RELATED APPLICATION BACKGROUND OF THE INVENTION This invention relates to pumps of the multiplex type. More particularly, this invention relates to a multiplex pump of the type in which the pumped fluid is precompressed prior to discharge.

In the oil industry it has been common in the past to utilize multiplex pumps designed to deliver pumped fluid at high pressures on the order of 15,000 p.s.i. or greater. It has been found that even the slight compressibility of this relatively incompressible pumped medium may result in a pulsating discharge pressure condition since a portion of the power intended to accomplish the discharge phase of each pumping cycle is inherently utilized to first compress the medium before it is brought to discharge pressure.

This discharge pattern is particularly undesirable where both high pumping pressure levels and very high delivery volumes are involved. The resulting pulsations could, under such conditions, subject the discharge conduits to severe vibrational forces. Thus, the pumping unit would be subject to stress conditions that might cause failure.

It would, therefore, be extremely desirable to provide a pump that is capable of delivery of a high volume of fluid at high pressure levels without being subject to pulsation problems.

To this end the present invention involves the provision of a precompression function in the pump that continuously serves to bring pumped fluid to a pressure approaching discharge pressure prior to actual discharge. Thus, a relatively smooth constant pressure output of the pumped fluid may be obtained.

It has been previously proposed to provide a fluid operated duplex pump (operable on a highly compressible fluid) with a precompression function in order to induce smoother discharge characteristics. This previously proposed pump is fluid operated in such a manner that one pumping unit is conditioned to undergo a discharge function while the other pumping unit undergoes both a suction and precompression function during the same time interval.

Although such a system may be satisfactory for some purposes, it may prove undesirable for a number of reasons.

For example, since all three functions associated with a pumping cycle (i.e. suction, precompression and discharge) are not simultaneously performed, there is an absence of constant pressure flow into the fluid end common to both pumping chambers. Therefore, the suction line or suction header common to the two pumping units is subject to pulsating flow that may have undesirable consequences similar to those intended to be eliminated in connection with the discharge function.

Moreover, the lack of simultaneous performance of all three functions also prevents constant pressure flow of power fluid into and out of pressure and reservoir headers common to the power cylinders constituting the power end of the pumping unit. Thus, pulsation problems may also be created at the power end ofthe pump.

It would, therefore, be highly desirable to provide a multiplex pump which provides non-pulsating suction flow as well as discharge flow. It would also be desirable to operate such a pump with power fluid that flows into and out of the power end of the pump without pulsation.

Another disadvantage of the previously proposed pump stems from the direct utilization of pressurized power fluid to produce the stroking of the power plungers in each direction and in each cycle phase. Thus, the cycle in each pumping unit is not inherently functionally dependant upon the cycle in the other unit. As a result, a phasing error in one stroke, e. g. overtravel or undertravel of the power plunger during precompression, cannot be corrected without intervention by an operator so that the error may be self perpetuating.

Furthermore, the previously proposed pumping unit does not utilize the power fluid in the rod end of the power cylinder to a most efficient advantage. During discharge and precompression this fluid in the rod end may be exhausted to a power fluid reservoir where the potential power of this fluid, which is pressurized by the rod displacement, is lost.

It would, therefore, be desirable to provide a multiplex pump with a precompression function and a self-regulating stroke control, as well as with efficient utilization of power fluid in the rod end of the power cylinder.

OBJECTS AND SUMMARY OF THE INVENTION It is therefore, a general object of the invention to provide a multiplex pump which obviates or minimizes disadvantages of the sort previously noted.

It is a particular object of the invention to provide an improved multiplex pump having a precompression function.

It is a further object of the invention to provide a multiplex pump which provides non-pulsating suction flow as well as discharge flow.

It is another object of the invention to provide such a multiplex pump which may be fluid operated with constant pressure flow of power fluid into and out of the power end of the pump.

It is a related object of the invention to provide a control valve assembly for such a multiplex pump which establishes constant pressure and flow of power fluid into and out of the power end of the pump.

It is still another object of the invention to provide a multiplex pump having a precompression function and means for automatically regulating the pumping strokes.

It is a related object of the invention to provide a multiplex -pump having a precompression function and in which the stroking in each pumping unit is responsive to the stroking in the other units.

It is yet another object of the invention to provide an improved multiplex pump which may be fluid operated by the use of separate power and control circuits.

A preferred embodiment of the invention intended to accomplish at least some of the foregoing objects comprises a multiplex pump having at least three pumping units each operable in a cycle including suction, precompression, and discharge phases, with the cycles of each unit being out of phase with one another. The fluid end of the pump terminates in a common discharge line which is in fluid communication with the discharge end of each pumping unit. Likewise, thesuction ends of each of the pumping units are in communication with a common suction line.

Operation of the pump according to the described cycle insures simultaneous performance of all functions associated with a given cycle to the end that constant pressure and flow of the pumped fluid occurs in the common suction line and in the common discharge line.

Each pumping unit is fluid operated by power fluid acting on a piston rod assembly extending between the fluid end and a power cylinder assembly at the power end of that unit. Each power cylinder assembly is connected through a control valve assembly to a common flow line communication with a source of pressurized power fluid and a second common flow line communicating with a power fluid reservoir. By simultaneous performance of the suction, precompression and discharge phases of the cycle, a substantially constant pressure flow of the power fluid to and from these common flow lines is provided.

The control valve assemblies each include two sleeve valves communicating with a common chamber, which in turn communicates with a power cylinder. When one sleeve valve of a given control valve assembly is in an open position and the other is closed, pressurized power fluid enters the associated power cylinder assembly to provide a discharge function in the fluid end of the associated fluid end cylinder assembly. When the sleeve valves are in a reversed position, a suction function is permitted resulting in discharge of the power fluid in the power cylinder assembly to the power fluid reservoir. During a phase of the cycle when both of these sleeve valves are in closed position, a precompression valve is opened, and power fluid is directed to the power cylinder through this precompression valve.

A separate control circuit is utilized to move the sleeve valves to their desired positions.

The rod ends of the power cylinders are fluid interconnected so that the functions in each pumping unit are performed in response to those performed in the other units. Also, a portion of the power circuit is interrelated with the control circuit to provide for self-correction of the stroke lengths in the power cylinder assemblies.

THE DRAWINGS Other objects and advantages of the present invention will become apparent from the subsequent detailed description thereof in connection with the accompanying drawings in which: 1

FIG. 1 is a side elevational view partially broken away of a triplex pump according to the present invention;

FIG. 2 is a top plan view of the pump illustrated in FIG. 1;

FIG. 3 is a front elevational view of the pump shown in FIG. I, illustrating the control valve assemblies associated with the power end of the pump and the interconnection of the control fluid manifold blocks; I

FIG. 4 is a partial cross-sectional view of one control valve assembly;

FIG. 5 is a cross-sectional view of a precompression valve employed in the control valve assembly of FIG. 4;

FIG. 6 is an exploded perspective view of the lower control fluid manifold associated with the control valve assembly of FIG. 4;

FIG. 7 is an exploded perspective view of the upper control fluid manifold associated with the control valve assembly of FIG. 4;

FIG. 7A is a cross-sectional view of the upper portion of the control fluid manifold in FIG. 7 and the check valves assembled therein;

FIG. 8 is a schematic illustration of a power circuit and a control circuit of the present invention;

FIGS. 9A1, 981 and 9C1 are respectively schematic illustrations depicting the control valve conditions, the power circuit flow, and the power cylinder assembly functions respectively associated with the first, second and third phases of the pumping cycle;

TABLES A1, B1 and Cl respectively provide an index of the valve conditions and cylinder functions depicted in FIGS. 9A1, 981 and 9C1;

FIGS. 9A2, 982 and 9C2 schematically illustrate the positions of the control circuit conditioning valves, the resulting movement of the control valves, and the control circuit fluid flow that accomplishes this control valve movement in the phases of the pumping cycle respectively associated with FIGS. 9A1, 98! and 9C1;

TABLES A2, B2 and C2 provide an index of the condition of the control circuit conditioning valves and an index of the control valve movement associated with these conditions as reflected in FIGS. 9A2, 982 and 9C2, respectively;

FIGS. 9A3, 983 and 9C3 schematically illustrate the trip ping of the cycling valves and the resulting control circuit fluid flow that causes the positioning of the control circuit conditioning valves illustrated respectively in FIGS. 9A2, 9B2 and 9C2; and,

TABLES A3, B3 and C3 provide an index of the tripped valve and its affect on the control circuit conditioning valves as respectively illustrated in connection with FIGS. 9A3, 983 and 9C3.

DETAILED DESCRIPTION General Summary Referring now to FIGS. 1 and 2, an overall view of a triplex pump 20 according to the present invention is there shown.

The pump 20 includes a fluid end assembly 22 comprising three substantially identical cylinders 24. The fluid end assembly is of the type utilized in the I-IT-400 pump series referred to on page 6 of the 1968 Sales and Service Catalogue" of Halliburton Services, Duncan, Oklahoma.

The internal passages 26 of each of the fluid end cylinders 24 are each in communication with a valved pump cylinder head 28 of the type more particularly described in US. Pat. No. 3,259,075, assigned to the assignee of the present invention. The disclosure of this patent is hereby incorporated by reference. Each of the cylinder heads 28 is provided with a conventional suction valve assembly 30 and a conventional discharge valve assembly 32. The discharge valve assemblies 32 communicate with a common discharge manifold 34, and the suction valve assembly similarly communicates with a common and conventional suction header (not shown).

Extending from the fluid end assembly 22 in a direction away from the chambers 28 is a power end assembly 38. The power end assembly 38 includes three substantially identical power cylinders 40 having internal passages 41. Each of these power cylinders 40 is in generally longitudinal alignment with one of the fluid end cylinders 24. A piston rod assembly 42 extends longitudinally into each power end cylinder 40 and the aligned fluid end cylinder 24. The ends of the piston rod assemblies 42 which extend into the chambers 26 of the fluid end cylinders 24 are provided by capped plunger ends, which function as pumping pistons 44 that are operable to bring about suction and discharge action in a conventional manner, and precompression action in a manner hereinafter more fully described.

The opposite ends, or power pistons, 46 of the piston rod assemblies 42 are in sliding and sealed engagement with the walls of the internal passages 41 of the power cylinders 40. In a manner hereinafter more fully described, power fluid acts on opposite faces 48 and 50 of the power pistons 46 to reciprocate the piston rod assemblies 42.

The power end assembly 38 and the fluid end assembly 22 are separated by a spacer frame assembly 51 which permits the fluid end piston rods, or plungers, 52 and the power end piston rods 54 to be separate members thereby facilitating maintenance operations. These rods 52 and 54 are each hollow, cylindrical members sealingly received in the fluid end cylinder passages 26 and the power end cylinder passages 41, as indicated at 55 and 56. If desired, a floating annular rod seal may be employed so as to allow the rods to operate slightly eccentric to the power cylinder bores, thereby eliminating the necessity of extremely accurate alignment between the power cylinders and the fluid end cylinders.

Extending longitudinally of and internally of each of these members are tie rods 57 which are joined at one end to the pistons 44 and 46 and at the opposite end with a cam actuator 58 to form, together with the rods 52 and 54 and their associated piston means 44 and 46, the integral piston rod assembly 42. The permissible stroke length of each piston rod assembly 42 is such that each cam actuator 58 is movable between a back position adjacent the power end cylinders 40 and a forward position adjacent the fluid end cylinders 24.

In a manner hereinafter more fully described, the cam actuators 58 are operable, in connection with cycling valves FP, to provide a signal that the piston rod assembly has reached its forward position. These cycling valves FP are mounted on the spacer frame 51 at that forward position by suitable mounting means, indicated at 59. The location of this mounting means is such that the lengths of the cam actuator 58 cooperate with the valves F P for a time sufficient to permit the necessary circuit functions to take place.

In a similar manner, at least one cam actuator 58 is operable, in connection with a stroke control valve HP, to provide a signal that its associated piston rod assembly 42 has reached its back position. This stroke control valve BP is also mounted on the spacer frame 51 at that back position by suitable mounting means 60.

At the end of the power cylinders 40 remote from the fluid end assembly 22, each power cylinder is in continuous communication with one of three identical control valve assemblies CV. To facilitate description of a pump of the present invention, the three control valves are hereinafter referred to as l-CV, 2CV and 3-CV, respectively. Similarly, the hereinafter described identical portions of the CV valve assemblies are differentiated by the prefixes 1-, 2- and 3-, as are the associated power cylinder assemblies 40.

The function of the control valves CV is to direct power fluid to and from the power cylinders 40 in a manner such that the power cylinders each operate on a suction, precompression, discharge cycle, each out of phase with one another.

In the discharge phase of the cycle in a given power cylinder 40, power fluid acts on the outer face 50 of a power piston 46 to transmit force through the piston rod 42 so as to cause the fluid end piston 44 to move to its forwardmost stroke position whereby fluid in the cylinder head 28 is expelled to the common discharge manifold 34. Prior to the discharge phase of the cycle, this fluid has been precompressed by power fluid acting on the power piston face 50 after passing through a precompression valve PR mounted on the control valve assembly CV. This precompression flow of power fluid causes the power piston 46, through the piston rod assembly 42, to move forward by in increment sufficient to compress the fluid to be pumped and thereby raise the pressure of the fluid to approach the discharge pressure.

Suction movement of each power piston 46 is caused by power fluid acting on the inner face 48 of the power piston 46. The internal passages 41 of the power cylinders 40 are fluid interconnected in a normally closed circuit by a suitable common conduit 61. Thus fluid in two rod ends of the passages 41, which fluid is displaced during precompression and discharge movement of the associated power pistons 46, is caused to flow through this common conduit 61 into the third passage 41 to act on the inner face 48 of the power piston 46 in that third passage 41.

In this manner, the suction, precompression and discharge functions are simultaneously and responsively performed, one function being performed in each fluid end cylinder 24. Therefore, constant pressure flow continually exists between the fluid end of the pump and the common suction header and common discharge manifold 34.

For purposes of accomplishing automatic stroke correction, as hereinafter more fully described, the common conduit 61 connecting the rod ends of the power cylinders 40 is in fluid circuit with a conventional accumulator 62 and with a source of power fluid through a normally closed filling valve (not shown in FIG. 1) hereinafter described. Also, a rod end relief valve 222 provides selective communication between the rod ends and a power fluid reservoir.

Also, as hereinafter described, the control valve assemblies CV which direct the power fluid, preferably water, are, in the illustrated embodiment, monitored by a separate control fluid circuit, preferably utilizing a different fluid such as oil, air or a combination thereof. It will, however, be apparent that controls other than a control fluid circuit (e.g. an electrical sensing arrangement) may be utilized to monitor the power circuit.

It will be appreciated that the elements of the control circuit, the control valve assemblies CV, the power end assembly 38, and the fluid end assembly 22, may all be mounted on a suitable common frame such as a skid 64. Preferably the mounting on the skid is designed to permit the fluid end cylinders 24 and the power end cylinders 24 to move longitudinally for a limited distance, during reciprocation of the piston rod assemblies 42, so as to relieve stresses on those members. Provision for such movement may be made by supporting the members 65 providing the connection between the spacer frame 51 and the cylinders 40 and 24 for limited sliding movement within suitable brackets as indicated at 66 and 68. These connecting members 65 may, in turn, be joined by tie bars 69. Detailed Structure:

The Control Valve Assembly Referring now particularly to FIGS. 3 and 4, the previously identified identical control valve assemblies 1-CV, 2-CV and 3-CV will be described.

From the partial cross sectional view of one assembly shown in FIG. 4, it may be seen that each assembly includes two pilot operated sleeve valves, I and D, which respectively provide for the inlet of power fluid to an associated power cylinder 40 and the discharge of power fluid from that cylinder. The sleeve valves 1 and D are generally vertically aligned and are slidably received in vertically aligned cylinders 70 and 72.

The bottom of the lower cylinder 70 communicates with a power fluid inlet elbow 74. Internally each elbow 74 is provided with a stepped bore indicated at 76, which continuously communicates with a source of pressurized power fluid. Referring to FIG. 3, it will be seen that the inlet elbows 74 of the control valve 1-CV, 2-CV and 3-CV communicate with a common passageway 75 which is turn provides the continuous fluid communication with pressurized power fluid.

The smaller internal diameter of the inlet elbow 74 at the step 76 is less than the external diameter of the lower end of the lower sleeve I so that downward movement of this sleeve beyond the step is prevented.

The upper end of the lower cylinder 70 and the lower end of the upper cylinder 72 are spaced from one another and are joined to a common, generally cylindrical, housing 78. This housing 78 is provided with a laterally facing opening 80 communicating with the piston end of a power cylinder 40 (see FIG. 1) and with a central chamber 82 within the housing. Within the chamber 82 is a slotted spacer 84 which is generally cylindrical and has an internal bore generally aligned with the lower cylinder 70.

This spacer 84 is provided, adjacent its lower end, with lateral slots 86 communicating with the central chamber 82. On top of the spacer and blocking its central bore from vertical communication with the central chamber 82 is a soft seat 88. The relationship of the outer diameter of the upper portion of the lower sleeve l and the inner diameter of the spacer 84 is such that when the lower sleeve 1 is in its uppermost position abutting the soft seat 88, fluid communication between the chamber 82 and the inlet elbow 74 is totally blocked. However, when the lower sleeve l is in its lowermost position adjacent the elbow shelf 76, fluid communication between the elbow inlet 74 and the chamber 82 is established by means of the lateral slots 86 and the sleeve valve 1.

The upper end of the control valve assembly is surrounded by a cylindrical discharge housing 90 defining a discharge chamber 92. The discharge chambers 92 of the control valve assemblies are in communication with one another and in continuous communication with a power fluid reservoir by means of connecting conduits 93 (FIG. 3) and 93', the former of which may by flexible to permit relative movement of the control valve assemblies during operation of the pump.

As seen in FIG. 4, the discharge housing 90, the central housing 78 and the elbow inlet 74 are suitably joined together by bolts 95. 

1. In a fluid operated multiplex pump, a power unit comprising, at least three power cylinders each having a power end for receiving and discharging power fluid, a control valve assembly associated with each of said power ends, each of said control valve assemblies comprising, inlet valve means for selectively establishing fluid communication between one of said power ends and a source of pressurized fluid common to each of said inlet valve means, discharge valve means for selectively establishing fluid communication between one of said power ends and a power fluid reservoir common to each of said discharge valve means, and precompression valve means for selectively establishing fluid communication between said source of pressurized fluid and one of said power ends through a fluid flow reducing means, and cyclically operable control means for selectively opening and closing said inlet valve means, said discharge valve means, and said precompression valve means for continuous flow between said power ends and both said said common source of pressurized fluid and said common power fluid reservoir.
 2. A power unit in a fluid operated multiplex pump according to claim 1 wherein: said cyclically operable control means provides said selective communication between each of said power ends and said common power fluid reservoir in two stages limiting the rate of decompression of said power ends.
 3. A power unit in a fluid operated multiplex pump according to claim 1 wherein said cyclically operable control means includes: pressurized power fluid flow diverting means for shifting a closed inlet valve means of one of said control valve assemblies to an open position while responsively shifting an open inlet valve means of another of said control valve assemblies to a closed position, with both said inlet valve means being open while shifting, and power fluid reservoir flow diverting means for shifting a closed discharge valve means of said another one of said control valve assemblies to an open position while responsively shifting an open discharge valve of a third one of said control valve assemblies to a closed position, with both said discharge valve means being open while shifting.
 4. A power unit in a fluid operated multiplex pump according to claim 3 wherein said cyclically operable control means further includes: means for operating said power fluid reservoir flow diverting means in response to operation of said pressurized power fluid flow diverting means.
 5. A power unit in a fluid operated multiplex pump according to claim 3 including: precompression power fluid flow diverting means operable to open a closed said precompression valve means of said third one of said control valve assemblies in response to opening of said discharge valve means of said another one of said control valve assemblies.
 6. In a fluid operated multiplex pump, a power unit comprising: at least three power cylinderS having rod ends interconnected in a normally closed fluid circuit, and each of said power cylinders being operable in a cycle including suction, precompression and discharge stages with the sum of the amounts of rod end fluid displaced during a complete discharge stage in one power cylinder and a complete precompression stage in another power cylinder providing the amount of rod end fluid needed to complete a suction stage in a third power cylinder.
 7. A power unit in a fluid operated multiplex pump according to claim 6 and including: accumulator means for removing fluid from said normally closed fluid circuit in response to a stroking error resulting in supply, to the rod end of one of said power cylinders in its suction stage of the cycle, of a greater amount of rod end fluid than that needed to complete that suction stage.
 8. A power unit in a fluid operated multiplex pump according to claim 7 including: stroke correction valve means for supplying fluid to said normally closed fluid circuit in response the sensing of an incomplete suction stage, in one of said power cylinders, caused by said stroking error.
 9. In a fluid operated multiplex pump, a power unit comprising: at least three power cylinders each having a blank end and a rod end defined by power piston and piston rod assemblies received therein, said power piston and piston rod assemblies being cyclically reciprocable in said power cylinders to perform suction, precompression and discharge strokes, power circuit means for cyclically supplying pressurized power fluid to said blank ends to initiate compression and precompression strokes, in said power cylinders and for exhausting power fluid from said blank ends during a suction stroke, rod end circuit means for providing a normally closed fluid circuit between said rod ends of said power cylinders, said rod end circuit means including accumulator means for removing fluid from said rod end circuit means in response to a stroking error of one of said power piston and piston rod assemblies, cam actuator means on each of said power piston and piston rod assemblies, cycling follower means, actuatable by each of said cam actuator means, for sensing the completion of a discharge stroke of each of said power piston and piston rod assemblies, and for initating cycle change, in response to actuation, by operating said power circuit means, and at least one stroke correction follower means actuatable by one of said cam actuator means, for sensing the completion of a suction stroke by one of said power piston and piston rod assemblies, and for initiating supply of fluid to said rod end circuit in the absence of actuation by said one of said cam actuator means.
 10. A power unit comprising: at least three power cylinders each having a power end for receiving and discharging power fluid, a control valve assembly associated with each of said power ends, each of said control valve assemblies comprising, inlet valve means for selectively establishing fluid communication between one of said power ends and a source of pressurized fluid common to each of said inlet valve means, discharge valve means for selectively establishing fluid communication between one of said power ends and a power fluid reservoir common to each of said discharge valve means, and third valve means for selectively establishing fluid communication between said source of pressurized fluid and one of said power ends through a fluid flow reducing means, and cyclically operable control means for selectively opening and closing said inlet valve means, said discharge valve means, and said third valve means for continuous flow between said power ends and both said common source of pressurized fluid and said common power fluid reservoir.
 11. A power unit according to claim 10 wherein: said cyclically operable control means provides said selective communication between each of said power ends and said common power fLuid reservoir in two stages limiting the rate of decompression of said power ends.
 12. A power unit according to claim 10 wherein said cyclically operable control means includes: pressurized power fluid flow diverting means for shifting a closed inlet valve means of one of said control valve assemblies to an open position while responsively shifting an open inlet valve means of another of said control valve assemblies to a closed position, with both said inlet valve means being open while shifting, and power fluid reservoir flow diverting means for shifting a closed discharge valve means of said another one of said control valve assemblies to an open position while responsively shifting an open discharge valve of a third one of said control valve assemblies to a closed position, with both said discharge valve means being open while shifting.
 13. A power unit according to claim 12 wherein said cyclically operable control means further includes: means for operating said power fluid reservoir flow diverting means in response to operation of said pressurized power fluid flow diverting means.
 14. A power unit according to claim 12 including: restricted power fluid flow diverting means operable to open a closed said third valve means of said third one of said control valve assemblies in response to opening of said discharge valve means of said another one of said control valve assemblies.
 15. A power unit comprising: at least three power cylinders having rod ends interconnected in a normally closed fluid circuit, and each of said power cylinders being operable in a cycle including retraction, limited extension and extension stages with the sum of the amounts of rod end fluid displaced during a complete extension stage in one power cylinder and a complete limited extension stage in another power cylinder providing the amount of rod end fluid needed to complete a retraction stage in a third power cylinder.
 16. A power unit according to claim 15 and including: accumulator means for removing fluid from said normally closed fluid circuit in response to a stroking error resulting in supply, to the rod end of one of said power cylinders in its retraction stage of the cycle, of a greater amount of rod end fluid than that needed to complete that retraction stage.
 17. A power unit according to claim 16 including: stroke correction valve means for supplying fluid to said normally closed fluid circuit in response the sensing of an incomplete retraction stage, in one of said power cylinders, caused by said stroking error.
 18. A power unit comprising: at least three power cylinders, each having a blank end and a rod end defined by power piston and piston rod assemblies received therein, said power piston and piston rod assemblies being cyclically reciprocable in said power cylinders to perform retraction, limited extension and extension strokes, power circuit means for cyclically supplying pressurized power fluid to said blank ends to initiate extension and limited extension strokes, in said power cylinders and for exhausting power fluid from said blank ends during a retraction stroke, rod end circuit means for providing a normally closed fluid circuit between said rod ends of said power cylinders, said rod end circuit means including accumulator means for removing fluid from said rod end circuit means in response to a stroking error of one of said power piston and piston rod assemblies, cam actuator means on each of said power piston and piston rod assemblies, cycling follower means, actuatable by each of said cam actuator means, for sensing the completion of an extension stroke of each of said power piston and piston rod assemblies and for initiating cycle change, in response to actuation, by operating said power circuit means, and at least one stroke correction follower means actuatable by one of said cam actuator means, for sensing the completion of a retraction stroke by one of said power pisTon and piston rod assemblies, and for initiating supply of fluid to said rod end circuit in the absence of actuation by said one of said cam actuator means. 